Seatbelt restraint systems for restraining an occupant in a vehicle seat play an important role in reducing occupant injury in vehicle crash situations. Seatbelt restraint systems of the conventional so-called “3-point” variety commonly have a lap belt section extending across the seat occupant's pelvis and a shoulder belt section crossing the upper torso, which are fastened together or are formed by a continuous length of seatbelt webbing. The lap and shoulder belt sections are connected to the vehicle structure by anchorages. A belt retractor is typically provided to store belt webbing and may further act to manage belt tension loads in a crash situation.
Seatbelt restraint systems which are manually deployed by the occupant (so-called “active” types) also typically include a buckle attached to the vehicle body structure by an anchorage. A latch plate attached to the belt webbing is received by the buckle to allow the belt system to be fastened for enabling restraint, and unfastened to allow entrance and egress from the vehicle. Seatbelt systems, when deployed, effectively restrain the occupant during a collision.
OEM vehicle manufacturers often provide seatbelt restraint systems with pretensioning devices, which tension the seatbelt either prior to impact of the vehicle (also known as a “pre-pretensioner”) or at an early stage of a sensed impact to enhance occupant restraint performance. The pretensioner takes out slack in the webbing and permits the belt restraint system to couple with the occupant early in the crash sequence. Upon the detection of a condition leading to an imminent impact or rollover, or in the event of an actual rollover, seat belt webbing is automatically and forcibly retracted by the pretensioner to tighten the seat belt against the occupant.
One type of pretensioning device is a pyrotechnic linear pretensioner (PLP), which can be implemented as a pyrotechnic buckle pretensioner (PBP) that is attached to a seat belt buckle. PLPs can also be attached to a webbing guide loop or seatbelt anchorage. Since both types pull a seat belt system component linearly to apply tension in the belt webbing, both PLPs and PBPs can be collectively referred to as a PLP. Examples of designs of PLPs and PBPs are provided by U.S. Pat. No. 6,068,664, which is hereby incorporated by reference. Typical PLPs have a pyrotechnic charge that is fired when a collision occurs, producing expanding gas which pressurizes a gas chamber within a tube, which forces a piston down the tube. The piston is connected with the belt system by a cable or strap. Stroking of the piston tightens or “pretensions” the belt against the occupant.
One design challenge with current linear and buckle pretensioners utilizing gas generators is efficient utilization of the gas volume produced by the generator. Typically, a significant volume of the gas produced by the gas generator leaks out of the device. Leak paths may be located around the piston, through the piston, and/or through the piston stroking tube around the opening through which the cable passes. A cable affixed to the piston typically has an uneven shape that is very difficult to seal around. Leaks paths allow gas to leak from the device, decreasing the pressure available for pretensioning the seat belt. Manufacturers have been forced to use larger gas generators to compensate for the loss. Moreover, manufacturing variations and the related lack of control of leakage paths can affect the repeatability of performance of the PLP.
Current PLPs require a fair amount of stroke distance for the piston, which must be able to travel approximately the distance of the webbing length that is desired to be retracted by the pretensioner. Accordingly, the required stroke distance places a constraint on interior vehicle design. Moreover, packaging space is a prime design constraint in incorporating PLPs. Typical PLPs utilize a round cylinder piston arrangement which makes the assembly bulky which limits ease of implementation, such as when it is desired to mount the unit behind interior trim panels or adjacent to vehicle seats.
In addition, the components of many presently available PLP devices are die cast, which compared with many other manufacturing processes, is expensive. Automotive component suppliers constantly seek reductions in costs of their products, and more efficient and less expensive methods of manufacture are desired.